Note: Descriptions are shown in the official language in which they were submitted.
2157~00
~ntern. Patent Appl. No. PCT/FR94/01547
Filing date: Dec. 28, 1~94
Applicants:: Furukawa Electric
Kawasaki Steel
Representative: Jean-Claude MOUGEOT, Pechiney
Other Applicants: Kaiser Aluminum, Pleasanton, CA
... Pechiney Rhenalu, 92400 Courbevoie, France
Applicant and Inventors: Binrun OH, Furukawa Electric
Yuichi Suzuki, Furukawa Electric
Kunihiko Kishino, Furukawa Electric
PROCESS FOR THE FABRICATION OF ALUMINUM ALLOY
HAVING HIGH FORM~BILITY
TECHNICAL FIELD
The present invention relates to a fabrication process to improve
the mechanical and forming properties of aluminum alloy sheet,
used particularly in automotive bodies.
STATE-OF-THE-ART
Automotive bodies are traditionally made from cold-rolled steel
sheet.
In the past few years, auto manufacturers have attempted to reduce
the weight of their models by studying the possibility of using
aluminum alloy of the Al-Mg-Si type in producing automotive
bodies, among other parts.
In this technology, the Al-Mg-Si alloy sheet is formed into an
element of an auto body after solution treatment followed by
natural aging into the T4 state. After forming, a hardening step
through aging ("bake hardening" heat treatment) applied during the
application or curing of the paints, imparts to the body the
required properties.
The main difficulty raised by the use of aluminum alloys in
automotive bodies is their insufficient formability. The
formability of aluminum alloys, and in particular that of Al-Mg-Si
alloys, therefore needs to be greatly improved.
Furthermore, aluminum alloy sheet suffers from low mechanical
properties compared to steel sheet. Manufacturers are therefore
interested in curing processes that, on the one hand, are
21~7000
efficient enough to impart to those sheets high mechanical
properties and, on the other hand,that require fairly short
treatment times and low temperatures to min;m; ze processing costs.
OBJECT OF THIS INVENTION
The present invention relates to a process for the fabrication of
aluminum alloy sheet having high formability, characterized in
that an aluminum alloy sheet composed of 0.3 to 1.7% (by weight)
Si, 0.01 to 1.2% Cu, 0.01 to 1.1% Mn, 0.4 to 1.4% Mg, less than
1.0% Fe and, the remainder, Al along with the inescapable
impurities, is subjected to a continuous solution treatment for at
least 3 seconds above 450C, followed by cooling between 60 and
250C and a preaging process for 1 minute to 10 hours at the
previous cooling temperature of between 60 and 250C.
The alloy may contain one or more elements, selected from among Cr
(between 0.04 and 0.4%), Zn (less than 0.25%), Zr (less than 0.4%)
and Ti (less than 0.2%).
DESCRIPTION OF THE INVENTION
The ranges of composition imposed in the invention on the
different alloying elements are justified by the following:
Si improves the mechanical properties by forming an Mg2Si
precipitate with Mg during the curing of the paint.
Its composition is selected in the 0.3-1.7% range by weight.
Indeed, below 0.3% its effect is insufficient and above 1.7%, its
formability after solution treatment decreases.
Mg improves the formability by forming a solid solution in the
matrix after the solution treatment. Furthermore, it improves the
mechanical properties by forming an Mg2Si precipitate with Si
during the curing of the paint. Its composition is selected in
the 0.4-1.4% range by weight. Indeed, below 0.4%, the increase in
mechanical properties is not sufficient and above 1.4%, the
formability after the solution treatment decreases.
Cu improves the mechanical properties by precipitating in
particular the q' and S phases as well as GP (Guinier-Preston)
zones, during the curing of the paint. Its composition is
selected in the 0.01-1.2% range by weight. Indeed, below 0.01%,
the increase in mechanical properties is not sufficient and above
1.2%, the corrosion resistance decreases.
Mn and Cr refine the grain size and the mechanical properties of
the matrix. Their composition is selected in the 0.01-1.1% and
the 0.04-0.4% range by weight, respectively. Indeed, at lower
concentrations, their effect is insufficient and above the upper
range, the formability after the solution treatment decreases.
~J
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Zn improves the mechanical properties. Zr and Ti refine the
microstructure. Their composition is selected to be lower than
0.25%, 0.4~ and 0.2% respectively. Above these values,the
formability will be too low.
Fe, a general impurity in aluminum, must be kept below 1.0% by
weight. Above this value, the beneficial effect of the invention
might not be realized.
Other impurities are also limited to less than 0.5 wt%. Above
this value, the benefits of the invention might not be realized.
Aluminum alloys of the A1-Mg-Si type are age-hardenable alloys:
aging induces precipitation of a hardening phase which increases
their mechanical properties. In the case of the Al-Mg-Si alloy,
the precipitation sequence is as follows:
Supersaturated solid solution --> GP zone --> Intermediate phase -
-> Stable phase
In the case of the solution/quenching/natural aging (T4) process,
the aging creates GP zones with precipitates left in excess after
the quench. These generate a clear improvement of the mechanical
properties.
The curing of the paint induces an artificial aging which in turn
induces the precipitation of an intermediate phase (age-hardening
phase) which allows for the optimization of the mechanical
properties of the alloy. The problem of this previous process
lies in the distribution of precipitates which, because they are
mainly concentrated in the GP zones during natural aging, prevent
the subsequent precipitation of the intermediate phase during
artificial aging and prohibit the achievement of optimal
mechanical properties. It is also not possible to directly form
the naturally aged alloy: The alignment of the GP zones with the
matrix phase (Al) deteriorates the formability to the extent that
it encourages failure along dislocations during deformation and
ultimately the build up of stresses at grain boundaries.
The present invention was conceived after analyzing these
different observations. It is characterized mainly by a permanent
holding at a temperature above 60C, without any incursion of room
temperatures, during the time spent between the solution treatment
and the final preaging.
The goal is to prevent the development of GP zones by maintaining
the temperature above 60C until the end of the preaging. This is
done in contrast with the previous process which included
precisely incursions at room temperatures, either during the
natural aging quench or until curing. These incursions were
responsible for the development of GP zones.
Once the sheet is preaged, it can be exposed to normal temperature
during forming and during painting and curing, without adverse
effects on mechanical properties.
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The fabrication process developed in the invention includes, after
the traditional melting, casting, homogenization and rolling of
the aluminum alloy described above, subjecting the alloy to a
continuous solution treatment of more than 3 seconds at a
temperature higher than 450C, followed by a cooling step to a
temperature between 60 and 250C at a rate higher than 100C/mn,
coiling at the cooling temperature (between 60C and 250C) and a
preaging step between 1 minute and 10 hours at the same
temperature.
The solution treatment improves the formability of the material by
inducing the temporary solubilization of elements such as Si and
Mg in the matrix. This later improves the mechanical properties
through the formation of fine precipitates of Mg2Si during the
subsequent curing step.
The solution heat treatment is applied for at least 3 seconds at a
temperature above 450C. Indeed, if the tçmperature and the time
are below 450C and 3 seconds respectively, the dissolution of the
elements (Si, Mg, etc.), and therefore the improvement in
mechanical properties during the subsequent curing, will not be
sufficient.
The cooling rate that follows the solution treatment must be set
higher than 100C/mn. Indeed, below 100C/mn, the precipitates
are not as fine, resulting in a mediocre formability and an
insufficient improvement in the mechanical properties during
curing.
The final temperature, for this cooling rate, is selected within
the 60-250C range. Indeed, if it is lower than 60C, GP zones
will form and if it is higher than 250C, a stable phase will
develop that will negatively affect formability and mechanical
properties.
Coiling of the material cooled between 60 and 250C, in the same
60-250C temperature range and the subsequent aging of 1 minute to
10 hours in the same 60-250C temperature range are designed to
allow the development of an intermediate phase, favorable to the
mechanical properties and formability of the alloy.
If their temperature is below 60C, GP zones form and if it is
higher than 250C, a stable phase will develop. Both will
negatively affect formability and mechanical properties.
The preaging time is to be set between 1 minute and 10 hours.
Indeed, below 1 minute, the intermediate phase is not precipitated
enough and GP zones might form when the temperature eventually
returns to normal. Above 10 hours, the overabundant intermediate
phase pushes the mechanical properties too high, resulting in a
lower formability.
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Finally, the present invention applies not only to the continuous
fabrication process mentioned above but also, with the same
effects, to the classical discontinuous processes.
PHOTOGRAPHS
Fig. 1 shows the microstructure of an example of aluminum alloy
sheet to which the invention applies.
Fig. 2 shows another example of the microstructure of an aluminum
alloy sheet to which the invention applies.
Fig. 3 shows the microstructure of an aluminum alloy sheet
processed according to the previous state-of-the-art methods.
Fig. 4 shows another example of the microstructure of an aluminum
alloy sheet processed according to the previous state-of-the-art
methods.
EXAMPLES
The invention will be better understood by virtue of the following
examples.
The aluminum alloys with the compositions shown in Table 1 were
made into ingots. The ingots were homogenized, hot-rolled at
400C and cold-rolled, by means of the usual methods, to yield 1-
mm-thick sheets. The sheets were subjected to a continuous
solution treatment for 10 seconds at 560C, followed by a heat
treatment using the conditions shown in Table 2, followed by a
preaging treatment between 1 minute and 10 hours at a chosen
temperature: 60C, 120C, 180C or 250C. Some of these sheets
were also finally subjected to a curing treatment (1 hour at
180C). Others were not.
For the purpose of comparison, sheets were also prepared using the
previous T4 process (solution and quench to room temperature).
Sheet samples were subjected to a tensile test, an Erichsen test
and formability limit test (punch test). The results are given in
Tables 3,4,5 and 6.
The tensile test was performed on tensile samples JIS No. 5. The
Erichsen test was conducted according to JIS Z2247A (measure of
the depth of the punch). The formability limit test consisted of
driving a round punch 33 mm in diameter into a lubricated blank,
of measuring the maximum blank diameter for which there was no
failure and of computing the ratio of the maximum diameter to the
punch diameter.
Table 3 gives the results obtained on sheets made of an alloy
having a composition in the range described in the invention and
subjected to a heat treatment as described in the invention. All
_ 21~7000
exhibit high properties: elongation between 22.8 and 34.0%;
tensile strength between 28.5 and 33.9 kg/mm2; yield strength
between 18.6 and 33.1 kg/mm2; Erichsen index between 9.1 and 12.6
mm; formability limit ratio between 1.90 and 2.53. In particular,
specimens that were not cured (heat treatments 1, 3, 5, 7) exhibit
high ductility, high Erichsen indices and high formability limits.
Cured samples (heat treatments 2, 4, 6, 8) exhibit lower
ductilities, Erichsen indices and formability limits but
noticeably higher tensile strength and yield strength. In other
words, such sheets first offer a good formability for shaping into
automotive body elements and acquire the required mechanical
properties during curing.
Table 4 gives the results obtained on sheets made of an alloy
having a composition in the range described in the invention and
subjected to a heat treatment described in the invention. All
exhibit characteristics noticeably lower than those of the sheets
presented in Table 3 and processed according to the invention:
elongation between 16.7 and 28.7%; tensile strength between 24.5
and 29.4 kg/mm2; yield strength between 16.7 and 20.8 kg/mm2;
Erichsen index between 8.3 and 8.8 mm; formability limit between
1.6 and 1.87.
Tables 5 and 6 give the results obtained on sheets made of alloys
having a composition outside the range described in the invention
but subjected to the process described in the invention. Again,
all exhibit characteristics sharply lower than those obtained on
sheets having the composition and prepared by the process
described in the invention: elongation between 16.4 and 28.6%;
tensile strength between 21.2 and 29.1 kg/mm2; yield strength
between 15.9 and 21.6 kg/mm2i Erichsen index between 8.2 and 8.8
mm; formability limit between 1.60 and 1.86.
Alloy C in Table 1 (Si 1.65%, Fe 0.08%, Mn 0.10%, Mg 1.38%, Zn
0.01%, Ti 0.02%, Al balance) subjected to heat treatment 3 from
Table 2 (Solution treatment for 10 seconds at 560C, cooling to
120C, coiling at 120C, preaging for 3 hours at 120C, no curing)
was selected as sample (a). The same alloy C subjected to heat
treatment 4 from Table 2 (Heat treatment of sample (a) followed by
curing for 1 hour at 180C) was selected as sample (b).
The same alloy C subjected to heat treatment 9 from Table 2
(Solution treatment for 10 seconds at 560C, cooling to 20C,
coiling at 20C, preaging for 3 hours at 120C, no curing) was
selected as sample (c). The same alloy C subjected to heat
treatment 10 (Heat treatment of sample (c) followed by curing for
1 hour at 180C) was selected as sample (d).
Samples (a), (b), (c), and (d) were photographed in plane {100}
using an electronic microscope (magnification x200,000).
Micrographs are shown in Figs. 1, 2, 3 and 4 respectively. We see
that the preaged sample exhibits a fine precipitation of an Mg2Si
intermediate phase (Fig. 1) and that the curing treatment makes
the precipitation even finer (Fig. 2).
2157000
Fig. 3 and 4 however show that cooling down to 20C prevents
precipitation of the intermediate Mg2Si phase, even if a subsequent
preaging treatment and a curing treatment are applied.
Thus, the process according to this invention offers great
industrial promise in that it allows the manufacture of aluminum
alloy sheet characterized by excellent formability and mechanical
properties.
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TABLE 1
Alloy Composition, by weight
Si Fe CuMn Mg Cr Zn Zr Ti Al
A0.350 11 0.20O.OS 0.43 - 0.02 0.01bal.
d
t
o
h
e
n
n
B 0.79 0.10 0.820.05 0.10 - 0.01 - 0.03 bal.
C 1.65 0.08 1.110.10 1.38 - 0.01 - 0.02 bal.
D 0.81 0.07 0.800.15 0.800.05 0.03 - 0.02 bal.
E 0.81 0.19 0.810.35 1.010.35 0.03 0.13 0.13 bal.
q
2157000
F 0.27 0.14 - - 0.73 - - - - bal.
c
m
t
e
x
m
G 2.10 0.05 1.04 0.100.93 - - - - bal.
H 0.93 0.06 2.06 0.052.01 - - - - bal.
I 1.63 0.16 - - 1.04 0.63 - - - bal.
1: According to the invention
2: Comparative example
1~
2157000
TABLE 2
Heat treatment
Solution Quench Coiling Preaging Curing
1 1560C-lOs 60C 60C 60C-lOh -----
(1040F-lOs)(140F)(140F)(140F-lOh)
2560C-lOs 60C 60C 60C-lOh 180C-lh
(1040F-lOs)(140F)(140F)(140F-lOh)(356F-lh)
3560C-lOs 120C 120C 120C-3h -----
(1040F-lOs)(248F)(248F)(248F-3h)
4560C-lOs 120C 120C 120C-3h 180C-lh
(1040F-lOs)(248F)(248F)(248F-3h)(356F-lh)
5560C-lOs 180C 180C180C-30mn -----
(1040F-lOs)(356F)(356F)(356F-30mn)
6560C-lOs 180C 180C180C-30mn 180C-lh
(1040F-lOs)(356F)(356F)(356F-30mn)(356F-lh)
7560C-lOs 250C 250C 250C-lmn -----
~1040F-lOs)(482F)(482F)(482F-lmn)
8560C-lOs 250C 250C 250C-lmn 180C-lh
(1040F-lOs)(482F)(482F)(482F-lmn)(356F-lh)
2 9560C-lOs 20C 20C 120C-3h -----
(1040F-lOs)(68F) (68F)(248F-3h)
10560C-lOs 20C 20C 120C-3h 180C-lh
(1040F-lOs)(68F) (68F)(248F-3h) (356F-lh)
11560C-lOs 20C 20C 250C-lmn -----
(1040F-lOs)(68F) (68F)(482F-lmn)
12560C-lOs 20C 20C 250C-lmn 180C-lh
(1040F-lOs)(68F) (68F)(482F-lmn)(356F-lh)
13560C-lOs T4 ---- ----- -----
(1040F-lOs)
14560C-lOs T4 ---- ----- 180C-lh
(1040F-lOs) (356F-lh)
1: According to the invention
2: Comparative example
1l 21~7000
TABLE 3
Treat Alloy El% Rm Ro 2 Erichsen Formabil
ment kg/mm2 kg/mm2mm ity
1 A 33.7 28.7 18.7 12.5 2.52
1 B 33.0 28.9 18.6 12.4 2.52
1 C 32.5 29.4 19.2 12.4 2.48
1 D 34.0 29.3 18.7 12.6 2.51
1 E 31.9 30.1 20.1 12.1 2.32
2 A 25.4 34.8 26.8 9.5 1.82
2 B 24.6 35.0 27.6 9.4 1.91
2 C 23.8 36.7 28.3 9.3 l.9O
2 D 23.4 37.2 28.9 9.3 1.90
2 E 22.9 38.9 31.0 9.2 1.90
3 A 33.5 28.5 19.3 12.4 2.52
3 B 32.9 29.4 18.7 12.3 2.49
3 C 32.8 29.8 19.8 12.3 2.48
3 D 32.7 29.7 20.1 12.3 2.49
3 E 31.8 30.5 21.8 12.1 2.31
4 A 25.8 34.8 27.6 9.6 1.92
4 B 24.9 35.6 28.6 9.4 1.91
4 C 23.7 36.4 29.9 9.3 1.90
4 D 23.9 37.6 31.2 9.4 1 91
4 E 23.7 38.7 32.4 9.4 1.90
~ 12 2157000
TABLE 3 cont inued
Treat- Alloy El% RmRo~2 Erichsen Forma-
ment kg/mm2 kg/mm2 mm
A 34,0 28,7 19,3 12,6 2,53
B 32,9 29,0 19,7 12,4 2,52
C 33,7 28,7 20,6 12,5 2,53
D 32,7 29,9 21,4 12,4 2,52
E 31,8 30,4 22,5 12,1 2,30
6 A 25,7 35,2 30,7 9,6 1,92
6 B 25,8 34,8 29,8 9,6 1,93
6 C 2~,6 36,7 31,5 9,4 1,91
6 D 23,8 38,0 32,7 9,3 1,90
6 E 22,8 39,0 33,1 9,1 1,90
7 A 33,7 28,6 21,0 12,5 2,54
7 B 32,9 Z9,7 20,4 12,4 2,52
7 C 32,5 28,7 22,7 12,3 2,49
7 D 32,8 30,1 20,7 12,3 2,48
7 E 32,7 30,2 22,7 12,4 2,51
8 A 25,6 34,8 28,9 9,6 1,92
8 B 25,7 35,6 28,5 9,6 1,92
8 C 24,8 36,4 28,4 9,5 1,91
8 D 23,7 37,5 29,5 9,3 1,90
8 E 23,7 38,6 30,7 9,3 1,90
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TABLE 4
Treat- Alloy El% Rm R0,2 Erichsen Forma-
ment kg/mm2 kg/mm2 mm bility
9 A 28,7 26,7 16,7 8,8 1,87
9 B 28,6 25,7 17,7 8,7 1,86
9 C 27,6 28,0 17,8 8,7 1,85
9 D 26,7 27,6 16,7 8,6 1,84
9 E 24,9 26,4 16,8 8,5 1,82
A 18,6 28,7 19,8 8,4 1,71
B 19,7 27,9 20,3 8,4 1,76
C 18,7 29,4 17,8 8,4 1,75
D 16,7 28,7 19,6 8,3 1,61
E 18,2 28,9 18,9 8,4 1,70
11 A 27,6 27,0 17,6 8,8 1,86
11 B 26,8 26,7 16,8 8,6 1,84
11 C 27,5 26,5 16,7 8,7 1,85
11 D 24,3 26,7 17,2 8,6 1,84
11 E 27,6 27,6 16,9 8,6 1,83
- 21S7000
., 1~
TAsLE 4 cont inued
Treat- Alloy El% Rm Ro~2 Erichsen Eorma-
ment kg/mm2 kg/mm2 mm
12 A 16,7 28,6 19,4 8,3 1,61
12 B 18,4 27,6 20,6 8,3 1,62
12 C 16,7 28,8 20,3 8,3 1,60
12 D 18,5 26,7 19,6 8,4 1,70
12 E 17,7 27,7 20,8 8,4 1,65
13 A 25,7 24,5 16,7 8,6 1,84
13 B 28,4 26,7 17,5 8,8 1,86
13 C 27,6 24,6 16,8 8,7 1,85
13 D 28,5 25,9 18,0 8,6 1,84
13 E 28,4 26,4 16,7 8,8 1,85
14 A 16,7 27,9 20,4 8,3 1,60
14 B 18,6 28,6 18,9 8,4 1,70
14 C 17,7 27,7 19,2 8,4 1,65
14 D 16,5 26,5 17,8 8,3 1,61
14 E 17,7 27,7 19,9 8,4 1,65
~15700.(~
TABLE 5
Treat- Alloy El% Rm R0,2 Erichsen Forma-
ment kg/mm2 kg/mm2 mm
1 F 28,6 23,9 17,6 8,8 1,86
1 G 24,3 24,6 16,9 8,5 1,82
1 H 25,9 25,8 18,8 8,6 1,84
1 I 27,6 24,6 17,8 8,6 1,83
2 F 16,4 26,9 20,6 8,3 1,60
2 G 17,6 27,8 19,7 8,4 1,61
2 H 16,5 26,6 18,9 8,3 1,60
2 1 17,6 27,7 17,6 8,3 1,61
3 F 25,3 23,2 18,6 8,6 1,84
3 G 24,4 22,6 17,1 8,5 1,82
3 H 27,6 25,2 17,6 8,6 1,83
3 I 25,8 24,6 18,2 8,6 1,84
4 F 16,6 27,8 19,7 8,3 1,60
4 G 18,5 26,9 20,0 8,4 1,61
4 H 20,1 28,8 18,9 8,5 1,80
4 I 17,6 27,7 18,0 8,4 1,61
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16
TABLE 5 cont inued
Treat- Alloy El% Rm R0,2 Erichsen Forma-
ment kg/mm2 kg/mm2 mm
S F 26,4 22,6 17,1 8,6 1,84
G 26,6 24,1 16,5 8,6 1,83
H 25,8 23,8 17,7 8,5 1,82
I 25,5 22,9 17,2 8,5 1,81
6 F 18,5 27,6 21,0 8,4 1,61
6 G 16,5 28,3 20,7 8,3 1,60
6 H 16,4 27,6 19,6 8,3 1,61
6 I 17,7 28,8 21,6 8,3 1,62
7 F 26,8 21,2 18,0 8,6 1,84
7 G 26,7 25,0 16,5 8,6 1,85
7 H 25,7 21,3 16,7 8,5 1,83
7 I 26,5 22,4 18,4 8,5 1,84
215 700 0
17
TABLE 6
Treat- Alloy E1% Rm R0,2 Erichsen Forma-
ment kg/mm2 kg/mm2 mm
8 F 18,8 27,6 20,3 8,4 1,60
8 G 19,3 28,3 18,9 8,5 1,62
8 H 17,7 29,0 19,2 8,3 1,61
8 I 18,6 27,6 18,7 8,4 1,60
9 F 26,9 22,8 16,5 8,6 1,84
9 G 28,0 21,6 17,0 8,7 1,85
9 H 26,9 22,3 16,6 8,6 1,84
9 I 27,3 22,0 16,7 8,5 1,83
F 17,6 28,6 20,5 8,3 1,62
G 19,9 27,8 19,6 8,4 1,61
H 18,6 28,6 18,9 8,4 1,60
I 19,6 27,7 19,9 8,5 1,62
11 F 27,6 23,4 16,7 8,7 1,85
11 G 27,2 22,5 16,3 8,7 1,84
11 H 26,4 22,6 17,4 8,6 1,84
11 I 25,8 24,3 17,9 8,5 1,82
215~003
18
TABLE 6 cont inued
Treat- Alloy E1% Rm R0,2 Erichsen Forma-
ment kg/mm2 kg/mm2 mm
12 F 19,6 28,6 20,9 8,4 1,61
12 G 18,8 29,1 20,5 8,4 1,60
12 H 17,7 28,6 19,8 8,3 1,61
12 I 19,6 27,7 18,9 8,5 1,65
13 F 28,6 23,1 15,9 8,7 1,85
13 G 27,6 22,2 16,8 8,6 1,84
13 H 26,6 24,0 17,6 8,5 1,82
13 I 25,5 23,3 16,8 8,5 1,84
14 F 19,7 27,6 20,1 8,4 1,61
14 G 16,8 28,8 19,7 8,2 ] ,60
14 H 17,8 27,6 18,4 8,3 1,60
14 I 19,6 26,6 19,7 8,4 1,61
